1. Field of the Invention
This invention relates to the field of air pollution control equipment, and more specifically to a preformed-spray scrubber having special utility for the removal of particulate and/or gaseous contaminants from a gas stream.
2. Prior Art
Many types of scrubber devices are presently available for removing various contaminants such as finely divided dust particles, gases and the like from a gas stream. One type of scrubber is called a preformed-spray scrubber. A preformed-spray scrubber collects particles or gases on liquid drops that have been atomized by spray nozzles. The properties of the droplets are determined by the configuration of the nozzles, the liquid to be atomized and the pressure to the nozzles. Sprays leaving the nozzles are directed into a chamber that has been shaped so as to conduct the gas past the atomized drops. In the past, horizontal and vertical gas flow paths have been used, as well as spray-entry flowing co-current, counter-current or cross-flow to the gas stream. In a typical situation, the relative velocity between the drops and the gas is ultimately determined by the terminal settling velocity of the liquid drops.
Ejector venturis are preformed-spray devices in which a high-pressure spray is used both to collect particles and move the gas stream. High relative velocity between the drops and the gas stream aids particle separation. Preformed-sprays have also been installed in venturi scrubbers which use a fan to provide high gas-phase pressure drop. Particle collection in these units results from inertial impaction on the drops. Efficiency is believed related to a complex function of drop size, gas velocity, liquid-gas ratio, and drop trajectories. There is often an optimum droplet diameter which varies with fluid-flow parameters. For drops falling at their terminal settling velocity, the optimum droplet diameter for fine-particle collection is around 100-500 .mu.m; for drops moving at high velocity within a few feet of the spray nozzle, the optimum is smaller.
Spray scrubbers are believed to be practically immune to plugging on the gas-flow side, but are subject to several problems on the liquid side. The liquid-gas ratio required is high; usually running 30-100 gallons per 1,000 cubic feet of gas treated, depending on efficiency. The sprays generate a heavy loading of liquid entrainment which must then be collected.
Most of the energy is put into the liquid sprays which can be configured so as to sweep the gas toward the exit. Gas-phase pressure drop (.DELTA.P) is therefore low or may even be positive.
In addition to the above problems, there are yet other problems associated with preformed-spray scrubbers. More specifically, it is desirable to limit the gas velocity so as to maintain the maximum relative velocity between the sprays and the gas stream. This requires that the cross sectional area for gas flow in the spray zone in the scrubber be fairly large. Such a large cross sectional area thus requires a large chamber, which is a significant shortcoming associated with prior art designs. Further, high gas velocities can cause distortion of the spray pattern so that there is not good coverage of the gas stream by the sprays. High gas velocity also causes increased entrainment of drops from the spray zone which thereby increases the performance required in the entrainment separator. In addition, high gas velocities generally adversely affect the capacity of typical entrainment separators. Finally, the transition from inlet ducting to the spray zone and from the spray zone to the outlet ducting presents a substantial problem in scrubber configuration and necessitates the dedication of a considerable fraction of the scrubber volume to this function.
The present invention is directed to an apparatus and related method which overcome a number of problems associated with the prior art. Further, the present invention has a relatively straightforward design making it easier to construct and install.